The present invention relates to systems for maintaining extremely low gas pressures during execution of industrial and scientific processes, particularly in regions which are continuously receiving a fresh supply of gas.
Many types of industrial and scientific processes are performed in a processing region which is evacuated to a very low pressure, of the order of several milliTorr (mT). Processes of this type include deposition and etching operations performed on semiconductor wafers with the aid of a plasma. In systems for carrying out such processes, a plasma is generated in a processing region which contains a processing gas maintained at a low pressure in the range of 1-100 mT, and frequently less than 10 mT. The gas will be ionized in the plasma and the resulting ions can be accelerated toward the wafer by suitable electric fields. During the course of the process, processing gas must be pumped out of the region at a high rate with a minimum of contamination by foreign materials, such as oil, that may be contained in the pumping equipment, and materials resulting from the processing itself, while fresh processing gas is supplied to the region.
In most processes of this type, the processing rate is dependent, inter alia, on the throughput, or rate of flow of processing gas molecules into and out of the processing region. This throughput, in turn, depends upon pumping speed and the fluid flow conductance between the processing region and the pump. The cost of processing a substrate is, in general, inversely proportional to the processing gas throughput.
The conventional technology employed to create low pressure levels of the order indicated above generally utilizes either one of the following two fundamental mechanisms: (1) increasing the momentum of gas molecules (hereinafter, references to xe2x80x9cmoleculesxe2x80x9d will be understood to encompass both atoms and molecules in those contexts where reference to both types of particles would be more technically correct) in a preferred direction and exhausting the gas through a valve or a baffle structure which inhibits reverse flow of gas; or (2) condensing the gas on specially prepared surfaces. Mechanism (1) is usually implemented with some type of piston, blower, or rapidly moving vanes which impart directed momentum to the gas by employing rapidly moving mechanical structures or streams of pumping molecules, such as molecules of mercury or readily condensable pumping oils. Mechanism (2) is commonly used in systems with low to moderate throughput requirements.
Turbomolecular pumps utilize mechanism (1) and are provided with rapidly spinning discs which impart directed momentum to gas molecules by colliding with those molecules. This mechanism is most effective for gas pressures which are sufficiently low that the mean-free-path of the molecules is longer than the dimensions of the pumping structures.
To establish low pump inlet pressures, of the order of 1-100 mT, employed in industrial plasma processes, it is presently the nearly universal practice to employ turbomolecular pumps as the first stage of a compound pumping system intended to pump large quantities of processing gas.
It has been found that the quality of processing operations of the type described above, and thus the quality of the finished semiconductor device, is dependent in large measure on the purity and composition of the processing gas and that these parameters can best be controlled if the flow rate of fresh gas into the processing enclosure is relatively high. The quality of the results produced by plasma assisted etching and deposition processes could be significantly enhanced if the gas throughput could be increased to a level between 3 and 5 times that presently utilized.
Although there are currently available high speed turbomolecular pumps which can achieve throughputs of the order of 5500 liters per second, at low inlet pressures, the highest capacity pumps that are currently available are also extremely expensive, as well as being less reliable than smaller pumps.
Moreover, even a throughput of 5500 liters per second has been found to be less than the optimum value for performing processes on wafers having a diameter of 200 mm, while achievement of optimum processing results on larger diameter wafers would require even higher gas throughputs. in general terms, the gas throughput required to achieve a certain processing result in terms of quality is proportional to the area of the substrate.
In addition, effective control of gas flow must allow for gas species that have a tendency to become attached to solid surfaces within the system. Such species include, for example, carbon compounds that are polymerized either by electrons or protons in the plasma. A plasma electron or proton flux can easily affix such materials to solid surfaces. Such materials may be subsequently released from the surfaces, perhaps in a modified form. The quality of any plasma assisted process of the type described above is dependent on the extent to which polymerized or otherwise modified materials can be prevented from being deposited on the substrate surface and this, in turn, depends on the extent to which such materials can be prevented from forming and/or remaining in the processing region.
Gas molecules which remain in the processing region for any significant time can be deposited on the substrate in a chemical form which is resistant to subsequent etching processes. As a result, these molecules will form defects on the substrate surface. The plasma pumping process can be greatly simplified if the concentration of organic and etching products in the gas phase can be made insignificantly small.
In view of the possible occurrence of such phenomena, it is apparent that the shorter the residence time of gas molecules in the processing region, the higher will be the quality of the product resulting from a series of etching and/or deposition processes.
In addition to the vacuum pumping technologies that have been used in connection with processing operations of the type described above, pumps using a plasma as an active element have been proposed. Plasma vacuum pumps would be capable of pumping a variety of gasses, including hydrogen and helium, with relatively high efficiencies, and are relatively immune to damage by solid or corrosive materials.
The operation of plasma vacuum pumps involves transforming three-dimensional flow of a neutral gas into one-dimensional flow guided by a magnetized plasma which may be magnetically compressed and guided through suitable baffle structures. Momentum can be imparted to the plasma as a result of various electromagnetic interactions and can be imparted to the neutral gas through collisions between molecules of the neutral gas and moving ions which have been accelerated and have greater momentum than background gas.
However, the potential benefits of using plasma vacuum pumps in plasma processing systems has not heretofore been realized to any significant extent. in particular, no solution has been proposed which combines efficient plasma generation with the creation of magnetic fields compatible with the plasma processing operation to be performed and suitable for channeling the plasma, as well as with a suitable mechanism for effecting pumping at pressures in the range which is of importance in such plasma processing operations.
The possibility of employing plasma vacuum pumping in plasma processing systems has been described, for example, in U.S. Pat. No. 4,641,060, which is issued to Dandl on Feb. 3, 1987. This patent discloses a plasma vacuum pump which does not employ any moving mechanical parts and which is capable of producing high pumping rates at gas pressures of less than 1 mT. A primary mechanism underlying this plasma vacuum pump is a magnetically guided flow of plasma ions and electrons through simple tubular baffle structures that restrict the flow of neutral gas molecules back into the region which is to be maintained at a low pressure. The pump disclosed in this patent appears capable of functioning effectively with magnetized plasmas at pressures below an upper limit determined by the spontaneous formation of electrostatic potentials that block the flow of plasma ions. xe2x80x9cMagnetized plasmasxe2x80x9d as used herein is a plasma in which the electron flow is magnetized, i.e., the electrons circulate around the magnetic field lines. While this form of plasma vacuum pump may prove suitable for some low pressure, magnetically confined plasma applications, it does not appear to be particularly suitable for typical industrial plasma processing systems.
It is an object of the present invention to pump ions out of a low pressure region at a high rate.
Another object of the invention is to provide a pumping system capable of achieving high pumping rates at a low cost.
A further object of the invention is to permit electrical control of the rate at which ions are pumped.
A still further object of the invention to effect ion pumping in a manner which promotes uniformity of processing across the surface of the substrate.
Yet another object of the invention is to control the profile, or distribution, in planes parallel to the substrate surface, of the speed at which ions are pumped above and around the substrate.
A further object of this invention is to use a plasma pump composed of an array of pumping cells to adjust the pumping speed and/or control processing region pressure with high response rates.
The above and other objects are achieved, according to the invention, by the provision of a novel plasma vacuum pump and pumping method for pumping ions from a first region, the ions possibly being generated by a plasma in the first region, to a second region when the plasma pumping cell is interposed between those regions. The plasma vacuum pump is constituted by: a partition member positionable between the first region and the second region, the partition member defining a plurality of conduits between the first and second regions; a source of free electrons in communication with the conduits; a plurality of groups of magnets, each group being positioned relative to a respective one of the conduits in a manner to provide lines of magnetic force that extend through the respective one of said conduits; and a plurality of electric potential sources, each source being disposed relative to a respective one of the conduits to create an electrostatic field which accelerates ions from the respective one of said conduits to the second region, wherein each of the conduits, together with a respective group of magnets and a respective electric potential source, forms a respective one of a plurality of pumping cells.
The magnetic fields produced by the magnets essentially influence the radial distribution of ions in the conduits and acts to trap electrons in the conduit. The trapped electrons act to prevent a positive space charge from developing within the conduits and give rise to electrostatic fields which accelerate positive ions from the first region into the conduits. While passing through the conduits, a certain proportion of the positive ions will combine with electrons to form neutral molecules. These neutral molecules will be carried by momentum into the second region. Ions which have not combined with electrons will be carried into the second chamber by momentum and the electrostatic field produced by the potential source.
The plasma in the first region, which may be the process plasma, ionizes a gas which has been introduced into that region, producing electrons that are magnetized and ions that are typically employed to carry out the process and that are to be pumped out of the first region so that a fresh supply of ions will be maintained in that region. The effect of the plasma on the electrons is to create a space charge that affects ion motion. According to one novel feature of the invention, the ions in the first region are given some directed energy, by the plasma and this directed energy aids the pumping action.
According to a further novel aspect of the present invention, the pumping cells are positioned relative to the processing region in a manner to increase the efficiency with which ions can be pumped out of that region. Preferably, the cells are disposed close to the plasma in the processing region so that use can be made of the highest possible plasma density to improve pumping efficiency. This configuration allows the average time that gas molecules remain in the processing region to be made extremely short.
A plasma vacuum pump composed of a plurality of cells according to the present invention may be disposed in a processing chamber which contains a substrate to be processed. It is desirable that the partition wall which carries the plurality of cells have an area which is as large as possible in order to increase the pumping rate and thus reduce the residence time of gas molecules in the chamber. However, this should be accomplished without significantly increasing the volume of the processing chamber since the residence time of gas in the processing chamber will increase as the chamber volume increases.
When the ions to be pumped have been generated in a plasma, ions adjacent the boundary of the plasma region will be influenced by the plasma sheath and ions in the vicinity of the pumping cells will experience some pre-sheath motion toward the cells. These ions will then be attracted into the cell conduits by the electrostatic fields created by electrons trapped in the conduits. The intensity of these electrostatic fields is determined essentially by the ambient ion and electron densities and the electric fields produced by the electric potential sources. The electrostatic fields extend through the conduits and act to accelerate ions from the first region, near the partition member, into the conduits.
In addition, momentum will be imparted to un-ionized gas molecules in the chamber by ions of gas molecules that have just entered the chamber and have a low, finite level of directed energy. Collisions between these ions and neutral species of exactly the same molecular structure and weight results in a very efficient, or resonant, transfer of energy. The efficiency of the transfer is inversely proportional to the speed of the faster one of the two colliding particles.
Gas molecules which are to be pumped need not be transported through any transitional structure in order to be removed from the system. In fact, processing gas molecules which have been injected into the processing region, or chamber, and which have not yet been ionized, will naturally travel in one of the following two possible ways:
(a) the more probable behavior of the molecules will be to bounce off of the surface of the substrate being processed and to be reflected in a cosine distribution about a normal to the substrate surface. The breadth of this distribution depends upon the fine surface features of the substrate;
(b) otherwise, the molecules may be adsorbed on the surface of the substrate and will then be re-emitted sometime later, possibly after having been converted to a different species having a different molecular weight. Such re-emission is also associated with a cosine distribution about the normal to the substrate surface.
The above-mentioned cosine distributions describe the pattern in which molecules are reflected or re-emitted in different directions from the substrate surface. The intensity, or quantity, of molecules emitted in each direction is proportional to the cosine of the angle formed between the direction and the substrate surface, which means that the intensity, or quantity, of emitted electrons will be greatest in the direction perpendicular to the substrate surface.
Control of gas flows within the processing chamber is enhanced if gas is introduced into the chamber in such a manner as to be propelled directly toward the substrate surface and if most molecules which have bounced off of the substrate surface are removed from the processing chamber by the vacuum pump after the first bounce off of the substrate surface. A simple calculation of molecular speed indicates that effective use of the kinetic energy of molecules to be removed requires that the number of bounces, or reflections, experienced by the molecules be minimized. To the extent that gas molecules can be pumped out of the processing chamber immediately after having bounced off the substrate surface, collisions with surfaces within the processing chamber can be minimized. Such minimization is advantageous because molecules which strike such surfaces may become attached thereto, and may subsequently be returned to the substrate surface in an altered form. Therefore, rapid removal of gas molecules allows the process being performed on the substrate to be controlled in a superior manner.
In order to affect ionized gas throughput, the electric field within each pumping conduit may be modulated so as to change the local accelerating field. This may be achieved by modulating the potential of each of the electric potential sources either individually or in groups, or by identically modulating the potentials of all of the electric potential sources. Adjustment of the plurality of cells making up the plasma pump in one of these ways may allow modulating the bulk pressure within the plasma chamber on a short time scale. The time scale for pumping response is directly related to the ion transit time which may be several microseconds to several milliseconds depending upon the transit distance through the pumping conduit. Since there are no moving parts within the pump (e.g., a gate valve that is typically used to control pressure), the time response is significantly improved over conventional pumping systems.
The pumping cells may be individually adjusted to affect the spatial distribution of the pressure, or pumping, field. The spatial distribution of pressure may also be controlled by appropriate design of the geometry and/or placement of the individual pumping cells. Specifically, the cells can be distributed in a nonuniform manner across the partition member.